Melanin is an omnibus term that describes a large family of natural and synthetic phenolic-quinonoid pigments of diverse origin and chemical nature. Natural melanins are generally differentiated by their origin, for example, bovine eye melanin, melanoma melanin, and sepia melanin. They usually occur in the form of granular particles and are secretory products of pigment-producing cells, the melanocytes. Synthetic melanins are named after the compound from which they were prepared via chemical or enzymatic oxidation (e.g., d,l-dopa., or 5,6-dihydroxyindole catechol melanin). In addition, melanins are classified according to their chemical structures into the insoluble black eumelanins (poly-5,6-indole quinones) and the alkali soluble red phaeomelanins (polydihydrobenzothiazines).
The study of melanins has led to the discovery of a number of biosynthetic pathways. For example, melanins can be produced by the oxidation of its precursors such as 1-dopa or tyrosine by a melanin-synthesizing enzyme, tyrosinase. Alternatively, melanin can be prepared chemically by auto-oxidation of 1-dopa or other substrates to melanin in the presence of atmospheric oxygen. Wilczok et al., Arch Biochem. Biophys. 23:257 (1984). Additionally, melanin can be prepared by a variety of electrochemical and photochemical methods from which individual steps of the melanization processes are identified and characterized. See, Crippa, et al. (1989), supra.
When melanin shows a characteristic UV absorption spectrum at a particular range of wavelengths, it is assumed that the melanin may exert a protective effect by absorbing or diminishing the energy generated by the correspondent wavelengths. In general, sunlight contains three types of ultraviolet wave lengths which may cause skin damage. UV A has the longest wave length and the lowest energy. Its wavelengths fall within the range of 320-400 nm, and such wavelengths can indirectly damage DNA by activating intracellular flavins or porphyrins in the formation of active oxygen species, e.g., hydrogen peroxide, hydroxyl radicals, oxygen radicals, etc. UV B falls within the range of 290-320 nm. It damages DNA both directly and indirectly. This type is probably the major causative factor in skin cancer. UV C has wavelengths between 200-290 nm. Its major effect is directly related to damage of cellular DNA.
Characteristics of light relevant here fall into two major categories: (1) luminous flux which is the effectiveness of light evoking brightness; (2) chromaticity which is referred to both the dominant wavelength that contributes to the actual color (i.e., hue) and the purity that establishes the saturation of color. The spectrum colors and their wavelengths are further categorized as the follows: violet (400 nm), blue (450 nm), green (500 nm), yellow (550 nm), orange (600 nm) and red (650 nm).
The enzymatic synthesis of melanin has been an area of extensive research due to its close resemblance to natural melanogenesis. The enzymes, tyrosinases, are widely distributed in nature and highly purified preparations have been obtained from mushrooms, (Lerch, K., Met. Ions. Biol. Syst., 13:144 (1981)), Neurospora crassa, Podospora anserina, potato tubers, broad beans, insect hemolymph and mammalian melanoma tumors. Lerch, K. and L. Ettlinger, Eur. J. Biochem. 31:427-437 (1972). It is generally agreed that these enzymes catalyze two types of reactions: the orthohydroxylation of monophenols to catechols, which is referred to as cresolase activity, and the dehydrogenation of catechols to o-quinones, designated as catecholase activity. Molecular oxygen is used for the hydroxylation reaction. For this reason, tyrosinase acting on a monophenol is referred to as a "mixed function oxidase". Hayaishi, O. in "Biological Oxidation" (Singer, T. P., ed.) p.581, Interscience Publishers, New York (1968).
Numerous investigations have revealed that the production of tyrosinase by a microorganism in a growth medium is regulated by such factors as the genetics of the microorganism, the composition of the medium, the growth temperature, the presence of biosynthetic inhibitors, the density of tyrosinase-producing cells and the presence of enzyme inducers. Katz, E. and A. Betancourt, Can J. Microbiol. 34:1297-1303 (1988). Some microorganisms are capable of producing extracellular tyrosinases which are synthesized intracellularly prior to their transport and secretion into the growth medium. Baumann, R., et al., Actinomycetes "The Boundary Microorganisms", pp.55-63, (Arai, T. ed), Tokyo Joppan Co. (1976).
The discovery of extracellular tyrosinase makes industrial pigment production by fermentation feasible. However, the high instability of the extracellular tyrosinase in the broth limits the recovery of active tyrosinase in these processes and hence limits production of melanin. Bauman, R. et al., (1976), supra. Moreover, the quantities and recovery of the secreted extracellular tyrosinases have been generally insufficient for a viable commercial process for the large-scale in vitro production of melanin. Loss of enzyme activity as a result of deactivation through the affects of oxygen, temperature and product intermediates has a severe affect on the yield of tyrosinase and, therefore, is responsible for the high instability and low yield of extracellular tyrosinase. Studies in Streptomyces antibioticus have shown that the kinetics of the loss of tyrosinase activity could be approximated by a first-order model, with a constant specific deactivation rate on the order of 0.1h.sup.-1. Proteolytic activity detected in stationary phase cultures suggested that proteolytic degradation of tyrosinase could be partly responsible for the loss of tyrosinase activity during this period of batch cultures, but tyrosinase deactivation was also observed during the growth phase when no proteolytic activity was detected. The specific deactivation rate was found to exhibit an Arrhenius dependence on temperature with an activation energy of approximately 20 kcal/mol. Growth of a culture using a two-temperature strategy along with an enriched growth medium resulted in a 2.5-fold increase in the amount of tyrosinase obtained over control cultures. Gardner, A. R. and T. W. Cadman, Biotechnology and Bioengineering 36:243-251 (1990). Improved processes for making tyrosinase and melanin are disclosed in PCT publication W092/00373 dated Jan. 9, 1992, which claims priority from U.S. Ser. Nos. 545,075, Jun. 29, 1990 and 607,119, Nov. 2, 1990, the disclosures of which are each incorporated by reference herein.
In addition to growth manipulation and temperature optimization, several microorganisms may be genetically engineered to further enhance their abilities to produce tyrosinases. These microorganisms include, but are not limited to species of Streptomyces, Escherichia, Bacillus, Streptococcus, Salmonella, Staphylococcus, and Vibrio. Many species of Streptomyces are capable of forming dark melanin pigments due to expression of tyrosinase from the mel gene locus. For example, the mel locus of S. antibioticus has been cloned and sequenced, Katz, E., et al., J. Gen. Microbiol. 123: 2703 (1983); Bernan, V. et al., Gene 37:101 (1985) and shown to contain two open reading frames (ORFs) that encode a putative ORF438 protein (MW=14,754) and tyrosinase (MW=30,612). ORF438 and tyrosinase are thought to be transcribed from the same promoter in S. antibioticus, and both genes are required for melanin production. Bernan, V., et al., (1985), supra. Based on genetic evidence, the ORF438 protein has been shown to function as a trans-activator of tyrosinase. Lee, Y.-H.W. et al., Gene 65:71 (1988). It has been suggested that the ORF438 protein is involved in tyrosinase secretion, or it may function as a metallothionein-like protein that delivers copper to apotyrosinase. Bernan, V.,et al., (1985), supra; Lee Y.-H.W., et al., (1988), supra.
The oxidation of a variety of N-terminal, C-terminal, and internal tyrosine peptides of melanin in the presence of mushroom tyrosinase has been studied spectrophotometrically. The spectroscopic patterns of these oxidations fall into three types: a dopachrome sequence, a dopa-quinone sequence, and a protein sequence. The dopachrome pattern is characterized by the formation of an intermediate oxidation product with absorption maxima at 305 nm and 480 nm. This absorption spectrum is exhibited by compounds of the aminochrome class, especially by dopachrome itself. The final oxidation product shows a maximum at 325 nm. This is very similar to the maximum at 319 nm exhibited by derivatives of 2-carboxy-5,6-dihydroxyindole, which is an intermediate in the enzymatic oxidation of 3,4-dihydroxyphenylalanine under conditions in which decarboxylation does not take place. Compounds which show the dopachrome pattern upon oxidation have blocked carboxyl groups and cannot undergo decarboxylation. It is accordingly probable that the development of the 325 nm absorption band is due to the accumulation of a derivative of 2-carboxy-5,6-dihydroxyindole. The dopaquinone pattern is characterized by the formation of an intermediate oxidation product with an absorption maximum at 390 nm. The protein pattern of oxidation, which is observed with a polypeptide which contains tyrosine may vary, depending upon the particular substrate. The distinctive absorption patterns, therefore, may be used to evaluate the positions of tyrosine in the peptide chains. Yasunobu, K. T., et al., J. Biol Chem. 234:3291-3205 (1959).
Similar spectrophotometrical studies have been conducted on melanin for the enzymatic oxidation of a single amino acid in the presence of tyrosinase. The process was observed to proceed in three chromophoric phases, the first corresponding to the formation of the red pigment, the second to an intermediate purple pigment, and the third to the formation of melanin. By comparison of the observed spectra of these products with those of known substances, it is possible to identify the intermediates during the process of melanin formation. Mason, H. S., J. Biol. Chem., 168:433 (1947); Raper, H. S., Biochem. J., 21:89-96 (1927).
Recently it has been shown that a number of enzymes can express their activity in reaction media where most of the water has been replaced by an organic solvent. Kibanov, A. M. Chemtech June., 354-359 (1986). For example, laccase purified from Trametes versicolor can oxidize 2,6-dimethoxyphenol and syringaldazine in hydrophobic solvents presaturated with water. Milstein et al., Appl. Microbiol. Biotechnol 31:70-74 (1989). In addition, the ortho-hydroxylation of aromatic compounds by the enzyme polyphenol oxidase such as tyrosinase suspended in organic solvents has also been found to be efficient. Doddema, H. J. Biotechnology and Bioengineering 32:716-718 (1988). However, some of the reported prior art methods require the enzymes to be immobilized on an inert surface or carrier prior to enzymatic reaction conducted in organic solvents.
In addition to tyrosinase, a number of other enzymes possessing a peroxidizing activity such as horseradish peroxidase, chloroperoxidase, milk peroxidase, cytochrome C peroxidase and microperoxidase have also been used to produce melanin in vitro. These methods generally require the inclusion of hydrogen peroxide as a substrate in the reaction mixture and, in some instances, the immobilization of peroxidase prior to the oxidation reactions. European Patent Publication No. 44 1, 689 A1. However, the in vitro synthesis of melanin under such harsh conditions has not been described or suggested when tyrosinases are used as the enzyme sources.
In light of the foregoing, there is a need for an improved method for the production of stable and highly active extracellular tyrosinase in commercially acceptable quantities. There is a further need to develop an improved in vitro process for the synthesis of melanins (hereafter polyphenolic polymers or "PPPs") generally and chemically modified PPPs specifically in the absence of a fermentation medium and microorganisms. This need is driven by the advantages that such an in vitro process would provide, i.e. eliminating the concern about using precursers in the reaction which would be toxic if used in vivo., and the avoidance of complications arising from the presence of cellular metabolites and debris.